1. Field of the Invention
The present invention relates generally to an objective having a long working distance, and more particularly to a microscope objective of high magnifications, which has a particularly long working distance.
2. Discussion of Related Art
Objectives for industrial microscopes are now increasingly required to have an ever-longer working distance. For instance, a working distance of at least 3 mm is required to observe a bottom pattern of recesses in an integrated circuit removed from an IC package. When an integrated circuit built on a substrate is observed, a long working distance is needed for the protection of the substrate because inspection of the integrated circuit must be carried out on substrates substrate of irregular shape.
Each year, integrated circuits are designed with ever finer patterns. For this reason, there is a growing demand for objectives capable of greater of than 100X magnification, which are rarely used.
When a dry objective is used to observe images with human eyes, the upper limit on the magnification of the objective is said to be about 100X. Even with greater than 100X, fine patterns may not be observed, because only a blurred image may be seen.
However, this holds only for observation with the naked eye. For recent semiconductor inspection, TVs or image analyses are often used. For these applications, it is required to raise the magnification of an objective so as to obtain the same resolution as in the case of observation with the naked eye, close to the low resolution of the image pickup sensor. When the contrast of a microscopic image is enhanced by image processing, it is possible to observe micro-structures that cannot be viewed with the naked eye.
Objectives having high magnifications and long working distances are known from JP-A's 4-40409, 8-190055 and 8-286112 and JP-B 4-26446. Some available objectives are capable of 250X magnification, but their working distance is about 1 mm at most. This working distance is insufficient for inspection of semiconductors while they are mounted on substrates.
The objective disclosed in JP-B 7-104488 has a working distance of about 15 mm at 100X magnification, which is enough for inspection of semiconductors while they are built on substrates. However, the magnification achieved is barely 100X.